1. Field of the Invention
The present invention relates to a black mask, a color filter and a liquid crystal display.
2. Description of the Related Art
A STN or TFT color liquid crystal display includes a color filter at a position opposite to a liquid crystal layer. The color filter has colored resins separated by a black mask having a plurality of apertures, and the visibility of the liquid crystal display depends upon the characteristics of the black mask. Conventional black masks are described in JP-A-8-179301, 8-36171, and 9-243801.
In a color filter for use in a liquid crystal display, it is required to reduce the optical reflectivity on the surface of a black mask, and the wavelength dependence of the reflectivity in the visible wavelength range in order to improve the visibility of a panel, that is, to make the change of the reflectivity in the wavelength range (wavelength dependence of reflectivity) small.
The black mask described in JP-A-8-179301 is so constructed that all of the layers contain the same kind of metal. That is, the black mask described in the above publication comprises a first antireflection film consisting of a chromium compound, a second antireflection film consisting of a chromium compound, and a screening film consisting of chromium, the films being successively formed. More particularly, the first antireflection film consisting of a chromium compound contains Cr, O, N, and C, the second antireflection film consisting of a chromium compound contains Cr, N, O, and C, and the screening film consisting of chromium contains only chromium metal. However, the wavelength dependence of the reflectivity on the surface of this black mask remains large, and the reflectivity is not reduced sufficiently.
The first antireflection film consisting of a chromium compound in the black mask described in the above JP-A-8-36171 contains chromium and oxygen as main components, the second antireflection film consisting of a chromium compound contains chromium and nitrogen as main components, and the screening film consisting of chromium contains chromium metal as a main component. However, there is a substantial difference in etching rates among these films. The etching rates depend upon the conditions of the film-forming process. Assuming the etching rate of the screening film mainly composed of chromium metal being subjected to wet etching by the use of an etchant such as ammonium cerium (IV) nitrate to be 1, the etching rate of the first antireflection film mainly composed of chromium and oxygen is generally 0.5 or less, while the etching rate of the second antireflection film mainly composed of chromium and nitrogen is generally 5 or more. Therefore, when the black mask of the above-described three-layer structure is subjected to wet etching, a precise pattern can be hardly formed due to the difference in etching rates among these films.
The black mask described in JP-A-9-243801 does not contain a chromium component. However, a precise pattern cannot be formed due to the difference in etching rate among the three layers.
An object of the present invention is to provide a black mask having improved etching characteristics at the manufacturing process, having low reflectivity in the whole visible wavelength range, and having a small wavelength dependence of reflectivity, which can be preferably used for a color filter and a liquid crystal display which can show clear pictures.